1. Field of the Invention
The present invention relates to thermosetting cationic latices, coating compositions prepared therefrom and a process for providing protective coatings for substrates, using said coating compositions.
2. Brief Description of the Prior Art
Cationic latices are known in the art and their method of preparation involving free radical emulsion polymerization of monomers in water, in the presence of cationic surfactants, is also known. The free radical emulsion polymerization procedure generally results in high molecular weight polymeric products which are normally in the range of 100,000 to several million. In this molecular weight range, the latices possess good mechanical properties. It has, however, been found that the latices leave something to be desired as to film properties relating to protective durability, i.e., the ability of coatings to maintain their integrity against chemicals, weathering, and the like.
In areas such as paper and textile manufacture where protective durability is not an essential service condition, cationic latices in their thermoplastic form have found a ready application primarily due to their inherent adhesion to anionic substrates. In areas such as protective coatings manufacture, where protective durability, particularly weathering, is an essential service condition, the cationic latices in their thermoplastic form are less desirable. In order to prepare good coatings, particularly protective coatings, with properties of outdoor durability, it is desirable to use thermosetting compositions. Coating compositions comprising thermosetting latices are, moreover, advantaged in that their application avoids problems such as air pollution, fire hazards and other problems which attend solvent-based coating compositions. The art-known thermosetting latices of commercial significance are basically anionic, and not cationic.
In developing the present invention, it has been found that there are a number of attendant problems that might have, in the past, hindered the development of cationic thermosetting latices of commercial significance. In the synthesis of substantially coagulum-free and stable cationic latices, strong acids such as HCl, HBr and the like have been used, to provide surfactant gegen-ions. It is believed that certain acids adversely affect some of the properties of the resultant cationic latices. It has, for example, been found that isocyanate curing agents, when formulated with the art-known cationic latices, impart ineffective cure, at ordinary cure temperatures.
In order to prepare effective thermosetting cationic latices, particularly those which will be useful as protective coatings especially in industrial applications, the above problems will have to be reduced or preferably avoided. Unfortunately, the art-known thermosetting cationic latices are, by and large, still attended by the aforesaid problems. Seemingly, the prior art has failed to discover the source of the aforesaid problem and/or to provide solutions therefor.
Co-filed application Ser. No. 305,590, filed Sept. 25, 1981 has disclosed the discovery of the source of the problem and the solution therefor. The thermosetting latices and the coatings described in that application are advantageously used on a variety of substrates. On ferrous metal substrates, however, the thermosetting latices, and compositions thereof, described therein tend to be corrosive as is manifested by flash rusting of the coated substrates. It is, therefore, desirable to provide thermosetting cationic latices, and compositions thereof, which can be applied to ferrous metal substrates to form corrosion-resistant coatings. This invention provides such latices and coatings prepared therefrom.